White clouded vitreous enamel



Patented Dec. 21, 1937 No Drawing. Application April. 8, 1933, Serial No. 665,193. In Austria April 20, 1932 3 Claims. (Cl. 106-36.2)

vsorbed during storage or resting. slip for gas cloudings which contains free cloud- This invention relates to the white-clouding of enamels by gas clouding.- Such processes form the subject of my earlier British patents, for instance Nos. 297,724, 297,725 and 297,737.

In the enamel industry it, is usual when employing the customary solid clouding means, such as tin oxide, zirconium oxide and the like, not to employ the enamelslip or barbotine immediately 7 after it has been made, but only when a considerable length of time has elapsed after the manufacture thereof.

Now, in order to obtain such stable or permanent enamel slips by employing gas clouding means, quite definite conditionshave to be fulfilled, and it is upon these conditions that the present invention is based.

The invention is based upon the discovery that in order to obtain slips which are stable when stored, the quantity of clouding agent required for the clouding must be completely adsorbed by the clay or enamel slip and must remain ad- An enamel ing means (that means to say clouding means which isnot bound to, or adsorbed by, any constituent of the enamel slip) no matter whether it be present from the outset or whether it becomes free (that is goes into solution) during storage, is namely not stable, and becomes worse, yielding, when employed in the clouding, retrogressions which are greater the longer the slip is stored or left at rest. Now, according to the present invention a slip which is capable of being stored or left at rest is obtained by obviating the formation of free clouding means during the storage or resting. The enamel slips are however the more capable of bearing storage or keeping,

the greater the quantity of the clouding agent that can be adsorbed by the clay or enamel slip. This invention therefore gives the rule, for the manufacture of permanent or durable slips, that for a given adsorption medium (clay or the like) those clouding agents are to be employed in which the quantity that is required for an optimum clouding is as far as possible below that quantity which can still be completely adsorbed in the slip, and is at most as great as the quantity which remains in an adsorbed condition in the slip during storage. According to the invention, therefore, the ratio of the completely adsorbable quantity of clouding agent to the quantity requisite for clouding must be at least approximately one to one, and the slip will be the more durable in storage the smaller the quantity of clouding agent that is required for clouding in proportion to the quantity of cloudingagent that can be adsorbed by the slip, and therefore the greater the. ratio of the quantity still completely adsorbable, by the enamel slip to the quantity required for the clouding. V

This invention'therefore presents ameans of manufacturing durable slips for gas cloudings. If it is found in the case. of a slip that it contains free clouding media, thisiis a sign that the slip is not durable when stored. In order toobtain 1 i I a slip that will bear storage the clouding agent must therefore be varied in such: a way that the ratio hereinbefore set forth isfulfilled. This ratio can be improved for a given clouding agent by increasing the quantity or adsorption me- "15' dium for example, that is, by increasing, the addi tion of clay. V

By this invention it is also'made ,possible to utilize for gas cloudings the advantagesof storage of the slip, A further feature of the invention is. based upon the discovery that highly polymerized hydrocarbons, particularly those of high molecular weight, or substitution products thereof, such as resins, pitches, tars, asphalts and's'o' forth, present special advantages as gas clouding agents; for thesesubstances can be completely adsorbed and retained by clay or by the enamel slip, and owing to the fact that these substanceshave a comparatively high decomposition temperature,

the evolution of gas is eff'ectedat a more favour- ,able'moment than with other clouding agents,

that is to say, not too soon, which is anadvan- 'tage for the regularity fofthe clouding.

As compared with intensively; coloured substances, such as dyestuffs, as clouding agents,

the highly polymerized hydrocarbons also have the advantage that they are uncoloured or atv least can only occasion slight coloration of the slip, as a result of which-the thickness of the 40 layer can be accurately observed and the application thereof is facilitated. s

i It is advantageous to employ solid highly polymerized hydrocarbons, particularly those of high molecular weight, and these preferably in-the form of a dispersion. V

In carrying out the process it is also-possible to employ products which contain such highly polymerized hydrocarbons or substitution products thereof, mixed with other substances, preferably with substances which do not have a cloud-f ing efiect, or at any rate only a weak clouding efiect.

The process according to the invention will 5 now be illustrated by detailed descriptions of specific examples. If, for example, benzidine is employed as a gas clouding agent, an addition of about 0.2 gram of benzidine to 1000 grams of enamel, and grams of clay is requisite for a suflicient clouding. Of this quantity of clouding agent the slip can only retain about 0.1 gram adsorbed. Consequently such a slip will not be permanent in storage. Such a slip must be used immediately after being made in order to obtain useful cloudings. The longer such a slip stands, the smaller becomes the adsorbed proportion of the clouding agent, and the smaller the clouding efiect becomes. i l r In order to obtain durable slips with benzidine, therefore, very large quantities of clay must be added, but these may already be injurious to the enamel.

For the manufacture of durable slips, however, organic dyestufi's, for instance, are suitable,

such as methylene blue, malachite green and the like.

If for example methylene blue is employed as a clouding agent, an addition of 0.3 gram of methylene blue, to 1000 grams of enamel, and 80 grams of clay yields a durable slip, giving ample cloud- For a useful clouding with malachite green from 0.25 to 0.3 gram of malachite green is necessary to 1000 grams of enamel material and 80 grams of clay. Such a slip is durable when stored.

The following examples refer to the use of highly polymerized hydrocarbons:-

(1) Grams Enamel, preferably a colloidal enamel 1000 Clay Gas tar (dried) 0.5 Saltpeter 0.4

are ground in a mill with about 450 grams of water.

(2) Grams Enamel, preferably a colloidal enamel 1000 Clay 100 Coal pitch (hard) 0.3 Saltpeter 0.4

are ground in a mill with 450 grams of water.

In these two examples the saltpeter may alternatively be omitted.

(3) Example of a colloidal enamel:

Grams 1. In a method of producing a metallic article having thereon a coating of clouded vitreous enamel having an opacity substantially greater than that which could be produced with the frit used and clay alone, and in which a substantial part of the increased opacity is contributed by gas bubbles retained in the enamel, incorporating in the enamel slip containing clay a gas-evolving agent which, upon the firing of the enamel, evolves gases which are retained in the enamel at the firing temperature thereof in the form of bubbles of such sizes and number as to contribute a substantial increase to the opacity, said gas-evolving agent being selected from the group consisting of highly polymerized hydrocarbons, substitution products thereof, dispersions of such highly polymerized hydrocarbons and substitution products thereof, resins, pitches, tars and asphalts.

2. The method of producing a metal article having thereon a coating of clouded vitreous enamel having an opacity substantially greater than that which could be produced with the frit used and clay alone, andin which a substantial part of the increased opacity is contributed by gas bubbles retained in the enamel, which comprises applying to the article a slip comprising an enamel frit, clay and a gas-evolving agent which, upon the firing of the enamel, evolves gases which are retained in the enamel at thev firing temperature thereof in the form of bubbles of such sizes and number as to contribute a substantial increase to the opacity, said gas-evolving agent being selected from the group con-' sisting of highly polymerized hydrocarbons, substitution products thereof, dispersions of such highly polymerized hydrocarbons and substitution products thereof, resins, pitches, tars and asphalts, and firing the slip on the article.

3. An enamel slip for clouded enamels for metalware to produce thereon a coating of clouded vitreous enamel having an opacity substantially greater than that which could be produced with the frit used and clay alone, and in which a substantial part of the increased opacity is contributed by gas bubbles retained in the enamel, comprising a ground mixture of enamel .frit, clay, water and a gas-evolving agent which, upon the firing of the enamel, evolves gases which are retained in the enamel at the firing temperature thereof in the form of bubbles of .such sizes and number as to contribute a substantial increase to the opacity, said gas-evolving agent being selected from the group consisting of highly polymerized hydrocarbons, substitution products thereof, dispersions of such highly polymerized hydrocarbons and substitution products thereof, resins, pitches, tars and asphalts.

IGNAZ KREIDL.

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